Tem Studies of Cement Hydration
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TEM STUDIES OF CEMENT HYDRATION
G.W. GROVES Department of Metallurgy and Science of Materials, Parks Road, Oxford OXI 3PH, U.K.
Oxford University,
ABSTRACT The application of transmission electron microscopy to the study of the early stages of hydration C3 S and the microstructure of very mature C3 S paste is reported. In the former case the effect of water-cement ratio appears to be very important. In the latter, a detailed evaluation of the microstructures of inner C-S-H, outer C-S-H and CH is possible and the relationship of these to models of cement hydration is discussed.
INTRODUCTION Microscopy has played an important role in shaping ideas about cement hydration. Amongst the various electron microscopy techniques, scanning reflection electron microscopy of fracture surfaces has been by far the most widely applied. It has the advantages of great convenience and good resolution of such morphological detail as may be exposed by the fracture path. It suffers from the drawbacks of not necessarily giving a representative view of the microstructure and of failing to give information about relatively dense regions where the fracture surface is flat. A technique which does not have these drawbacks is backscattered electron imaging of polished sections of cement pastes or concretes and this holds much promise, especially for quantitative phase analysis [1]. However the technique which has the greatest potential for high resolution, both of microstructural detail and of microanalysis, is transmission electron microscopy (TEM). In this paper we consider the application of this technique to hydration at two extremes. Observations of very early stages of hydration of C3 S are discussed and the microstructure of a very mature, 26 year old C3 S paste is. evaluated.
EARLY HYDRATION Many observations have been made of hydrate gel formed during the early stages of C3 S hydration. Double and Hellawell [2] observed gel coatings formed after 3 hours on grains in a wet cell in a HVEM, and this was taken as evidence for a membrane surrounding the grain and halting further reaction until disrupted by osmotic pressure at the end of the induction period [3], an idea developed also by Birchall et al [4] and originally suggested by Powers [5]. In this experiment the water-cement ratio was high, probably in excess of 10. Copious reaction product on C3 S thin foils immersed in the supernatent liquid of a 2:1 water-cement paste was reported by Groves [6]. Ings et al [7] observed a 5 pm thick hydrate layer formed on a single crystal of C3 S after 30 mins in a water-cement ratio of 2. This crystal, however, appeared to be unusually reactive and a suspicion of enhanced reactivity also rests on the thin foils observed by Groves, since they were prepared by ion-beam thinning, which might activate the surface of the C3 S. Thin flakes or foils were observed in a HVEM wet cell after 3 hours hydration by Jennings et al [8] at a water-cement ratio of 0.47, but it is not clear whether a continuous coating or membrane existed. It appears that the
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